1. Field of the Invention
The present invention relates to a multilayer piezoelectric device.
2. Description of the Prior Art
In diesel engines, a fuel is injected into each cylinder and compressed to a high pressure to induce autoignition, and the engine power output is controlled by adjusting the fuel injection level. Therefore, in diesel engines, the role of the fuel injection system which feeds the fuel into the cylinders is important. The increasing use of diesel engines in recent years is largely owing to the developments in so-called common-rail fuel injection system. The future increased use of diesel engines is expected to be backed up, in particular, by employing piezoelectric devices in lieu of solenoid devices as the drives for opening and closing injection nozzles. This is because piezoelectric devices can realize the opening and closing of injection nozzles more quickly and with higher precision than solenoid devices and, therefore, can feed more appropriate amounts of a fuel to cylinders with better timing. The best combustion conditions for the fuel can be thus established and thus provides marked effects as decreases in the amount of hazardous substances such as particulate matters, nitrogen oxides, carbon monoxide and hydrocarbons contained in the exhaust gas, fuel consumption improvement, engine noise reduction, and engine output improvement.
Piezoelectric devices applied to fuel injection systems has been utilized as power sources for driving injection nozzle opening and closing mechanisms generating displacements and stresses and are mostly used in a state of being contained in cases made of a metal or the like. Generally, multilayer ones are used as the piezoelectric devices.
In multilayer piezoelectric actuators comprising internal electrodes offset alternately, there are an active region (expanding and contracting regions) and inactive regions (regions neither expanding nor contracting) in the device inside. Great stresses thus build up at the boundaries between the active and the inactive regions, so that there is the possibility of stress-due cracking occurring and causing external electrode breaking as well. It is thus demanded that the durability of external electrodes be improved. For that purpose, it is conceivable to simply increase the strength of external electrodes to thereby prevent them from being cut apart. However, when the strength of external electrodes is high, the driving of the devices may be inhibited in some instances, with the result that the displacement efficiency may be reduced.
Japanese Patent (Kokai) Publication No. H07-226541A describes that a underlying layer for the external electrodes is bridged to secure conduction.
Japanese Patent (Kokai) Publication No. H10-229227A describes that a single-layer reinforcement layer is joined to the external electrodes in offset type devices.
The following documents also describe the joining of external electrode reinforcing layer in offset type devices:
Japanese Unexamined Patent (Kohyo) Publication No. 2003-502869A;
Japanese Unexamined Patent (Kohyo) Publication No. 2003-502870A;
Japanese Unexamined Patent (Kohyo) Publication No. 2003-503859A;
Japanese Unexamined Patent (Kokai) Publication No. 2002-171004A.